1. Field of the Invention
The present invention relates in general to a liquid gauge for measuring the quantity of a liquid, and more particularly to a so-called capacitance type liquid gauge which detects the quantity of a liquid existing in a container by disposing a pair of electrode plates in the container and measuring the capacitance established between the electrode plates.
2. Description of the Prior Art
A so-called capacitance type liquid gauge has been used in motor vehicles for measuring the quantity of fuel or gasoline existing in a fuel tank of the vehicle. This type gauge is constructed by employing the advantageous phenomenon in which the capacitance established between two parallel electrode plates disposed in a liquid container varies with increase or decrease of the liquid in the container. The capacitance C between the paired electrode plates is the sum of the capacitance established between the respective portions thereof of the plates which are submerged in the liquid and the capacitance established between the remaining portions thereof which are exposed to air, and the total capacitance C is expressed in the following equation. ##EQU1## where E.sub.L is the dielectric constant of the liquid, E.sub.A is the dielectric constant of air, d is the distance between the two electrode plates, S is the effective surface area of one electrode plate, and x is the ratio of the liquid-submerged portion of the area of one electrode plate to the effective surface area S. As is understood from this equation, when the liquid level in the container rises or lowers, the value x changes causing variation in the value of the summed capacitance. By suitably processing the variation of the summed capacitance, it is possible to indicate the liquid level in the container on a meter which may be either of analog type or digital type.
In practical liquid gauges based on the above described principle, it is popular to utilize the variation in the capacitance C to vary the frequency of a pulse signal generated by a CR oscillator which includes a resistance R connected in series with the capacitor constituted of the two electrode plates to determine a time-constant. The hitherto used liquid gauges of the type mentioned above exhibit a satisfactory performance in detecting the quantity of liquid especially when the liquid is of a type having relatively small dielectric loss, such as gasoline.
Apart from the above, for dealing with the energy crisis, it is currently proposed to use as a substitute for gasoline a so-called gasohol, which is a mixture of gasoline and alcohol. However, the hitherto used capacitance type liquid gauge fails to exhibit its normal performance against such gasohol because it has a dielectric loss considerably greater than that of gasoline due to addition of alcohol.